The invention relates to high pressure metal halide arc discharge lamps and is especially applicable to such lamps containing sodium iodide.
High pressure metal halide discharge lamps generally comprise a tubular fused silica arc tube containing an ionizable fill and having a pair of main thermionic electrodes in the ends. The electrodes are supported by inleads which include a thin molybdenum ribbon portion extending hermetically through a pinch seal in the end of the lamp. Generally a starter electrode is disposed in the arc tube adjacent one of the main electrodes to facilitate starting. A discharge can be ignited across the short gap between the starter electrode and its adjacent main electrode at a much lower voltage than is required to ignite a discharge across the longer gap between the two main electrodes. Once the discharge has ignited, the ionized gas decreases the resistance between the two main electrodes and the discharge changes over into an arc between them.
The ionizable fill of metal halide lamps comprises mercury, an inert starting gas such as argon, plus one or more metal halides having useable vapor pressures and a desirable emission in the visible spectrum. One well-known metal halide charge comprises the iodides of sodium, thallium, and indium. Another well-known charge comprises the iodides of sodium, scandium, and thorium. The addition of sodium has from the beginning been troublesome because sodium ions migrate out of the arc tube during lamp operation. As sodium is selectively lost and freed iodine is left behind, the lamp spectrum deteriorates through loss of the sodium radiation and the operating voltage rises, ultimately resulting in lamp failure.
It was found that sodium loss could be greatly reduced by changing the mount frame or harness which supports the arc tube within the outer envelope from the conventional side rod construction to a divided mount construction. In the former, the so-called side rods of the frame carry current to the electrode at the opposite end of the arc tube from the lamp base. In the latter which is a reversion to an integral mount disclosed in U.S. Pat. No. 2,888,585 -- Martt et al., the long side rods are eliminated and the mount is in two sections, one extending from the stem of the outer envelope to the arc tube and the other extending from the dome end of the outer envelope to the arc tube. In Electric Discharge Lamps, MIT Press 1971, John F. Waymouth explains the improvement as follows. Since the lamp operates on alternating current, the side rods alternately have a positive and a negative potential with respect to the surface of the arc tube. The side rods are bathed in ultraviolet light from the arc tube and as a result emit photoelectrons. When the side rods are negative, some of these photoelectrons drift to the outer surface of the arc tube, charging it up negatively. On the other half cycle, when the side rods are positive, there is no return photo current because silica is a very poor photo emitter. The resulting negative charge on the arc tube causes positively charged sodium ions to move through the fused silica to the outer surface where they evaporate off. When the side rods of the arc tube mount are eliminated, a major source of electrons is removed.
However even with the divided mount construction, an appreciable loss of sodium continues to take place. The object of the invention is to further reduce the loss of sodium from metal halide lamps.